Dictionary.net

Sometimes gives rise to alarming symptoms, even to abolition of the function of the brain, yet without any sensible organic disease. Slight concussion of the brain, called stunning, consists in vertigo, tinnitus aurium, loss of memory, and stupefaction; all these being temporary. When more severe, there is instant loss of sensation and volition, vomiting, the patient being as if in a sound sleep, but there is no stertorous breathing. Pulse variable, generally more rapid and feeble than in compression; extremities cold. Little can be done here, till reaction has occurred: after this, the case must be treated according to general principles, - by bleeding, blisters, cold applied to the head, &c. After severe concussion, a patient, although apparently well, is not safe till some time after the accident.

Effect of severe head-injury.